"Image-based spam is definitely one of the techniques employed to get around legacy technologies which is why spam filters look at more than image content for traffic patterns associated with sources and use reputation databases.

"A combination of technology needs to be used because the spammers know how to get around some products."

Paul Ducklin, Sophos Australia and New Zealand head of technology, said image-spam is becoming a more prevalent from of spam because it defeats older, simpler spam classifying engines.

Ducklin said any product relying solely on text-oriented classification will struggle because of the lack of words - Bayesian filters also have the same shortcomings.

"This is not anything new and there are lots of tools available to construct images, put background speckles in it and even use multilayered images in a GIF file across three different layers so when superimposed the message is legible," Ducklin said.